The cemented carbide is a type of metal ceramic material produced from hard compounds of refractory metal and bonding metal through the powder metallurgy process. The cemented carbide material contains abundant valuable metals, such as chromium, tantalum, titanium, tungsten, cobalt, nickel, molybdenum etc.
China has a shortage of cobalt resources and needs to import a large amount of cobalt each year. Although tungsten resources are abundant, with the large amount of mining in recent years, its storage and exploitation are decreasing day by day while the number of waste cemented carbide is increasing, and some waste cemented carbides may still have a far higher content of tungsten and cobalt than industrial raw materials. Due to the content of tungsten can be, up to 40%-95%, recovery of tungsten and cobalt resources can obtain enormous economic and social benefits. Therefore, recycling waste cemented carbide has significant importance for protecting and utilizing the existing mineral and ecological resources. The major recycling methods include nitrate melting method, zinc melting, thermal reduction method of carbon monoxide, mechanical-crushing method and selective electrochemical dissolution method.
Nitrate melting method is the earliest industrialized method for recycling waste cemented carbide. However, it requires a high corrosion resistance of the equipment. During the recycling process, harmful gases such as NO and NO2 are being released, destroying the work and ecological environment badly. Zinc melting is the most widely adopted method in waste cemented carbide treatment, however, the technology still has many disadvantages in the recycling process, such as high energy consumption, complicated equipment, and residual zinc existing in the products. The thermal reduction method of carbon monoxide is a method of oxidizing and calcining waste cemented carbide in air to form WO3 and CoWO4 composite oxide, then the WC/Co composite powder was obtained by carbothermal treatment. However, this technique is encountered with many difficulties such as high energy consumption and CO or CO2 emissions into the environment. Because the cemented carbide has high hardness and strength, it can be treated by mechanical-crushing method in order to recycle cemented carbide powder; however this technology needs advanced abrasive equipment and can easily cause secondary pollution in crushing process. Selective electrochemical dissolution is one of acid solution electrolysis process. The method includes the steps of electrochemical dissolving cobalt or nickel into the electrolyte, filtering, precipitating, calcining reduction and obtaining cobalt or nickel powder. While, anode slime was used as raw material of producing cemented carbide, by ball-mill treatment. This method is simple in operation, but it also has other problems, such as that the anode passivation will increase period of recovery, and that waste liquid treatment will increase the cost of recycling waste cemented carbide and so on.
The purpose of the metal-thermal reduction is to obtain pure metal. In other words, the metal-thermal reduction is a chemical reaction method for preparing metal element by using active metal reductant to replace the metal in another inactive metal compounds. And the molten salt metal-thermal reduction is a method for reducing and preparing metal element or alloy material by metal-thermal reduction. The molten salt metal-thermal reduction has aroused widespread concern because it has the advantages of short process, low energy consumption, simple equipment, environment-friendly and so on.
In the molten salt with CaCl2) reagent, the niobium powder with content above 99.5% can be obtained by Okabe et al. through magnesiothermic reduction process using Nb2O5 as the raw materials. Ryosuke at al. studied calclothermic reduction process to prepare niobium powder from Nb2O5 in saturated calcium molten salt system. In the same year, the calciothermic reduction process for producing niobium powder using Nb2O5 as raw material has been researched by Okabe of al. using electrochemical methods, and the pure niobium powder was successfully obtained. In the CaCl2) molten salt, Shekhter et al. focused on the research of the gas metal thermal reduction process using Ca or Mg vapor to reduce powdered rare-metal oxide. Moreover, the reaction time of preparing metal Nb powder by calciothermic reduction of Nb2O5 was studied in detail by Baba et al. The molten salt calciothermic reduction completes transformation of preparing single metal Nb powder from direct reduction of Nb2O5 powder (or body) by calcium steam to direct reduction of Nb2O5 in molten salt thinner by calcium atom, and the high-purity Nb nanopowder was successfully obtained. Afterwards, in the molten salt with CaCl2) system, a series of studies on Nb or Nb—Al intermetallic compound products were carried out by Hongrnin Zhu et al. through sodiothermic reduction process, and the Nb or Nb3Al/NbAl3/Nb2Al—NbAl3 nanopowder was successfully obtained. Above studies show the metal-thermal reduction method has the advantages of shortprocess, simple equipment, and the adjustable production scale.